What is Stopping Sight Distance? 6 Important Points

Stopping sight distance is one of the several kinds of sight distance used in highway or street design. It simply means the minimum forward distance a vehicle driver needs (for a particular posted speed) to see in order to make a decision to stop the vehicle before colliding or hitting other vehicles or road users or road assets on the road.

Inadequate sight distance can adversely affect road user safety or roadway operations.

What is the Stopping sight distance along the highway?

Stopping sight distance (SSD) is the sight distance accessible along the highway at any given spot and it enables the driver to slow down or stop the vehicle (which is running at a specified road speed) safely without a crash when the driver observes an unexpected obstruction on the road.

What is PIEV Theory?

1) Perception 2) Intellection 3) Emotion 4) Volition PIEV is the amount of time it takes a driver to react to a hazard. PIEV means PIEV time - perception, intellection, emotion, and volition. Before we can stop an automobile, four specific areas of activity need to happen. The total reaction time splits up into four categories in accordance with the PIEV theory.

Perception Time:

It is the time required to get a sense with the help of the eyes and or ears of the driver and transmit the information to the brain. In simple terms, it is the time required to perceive a real-time situation or an object such road user, vehicle, etc.

Intellection time:

It is the time required by the driver to understand the real-time situation and compare it against various available options before taking the action.

Emotion time:

It is the time elapsed during the emotional and other mental trouble such as anxiety, annoyance, or other forms of emotional feeling.

Volition time:

It is the time required by the driver for making the final action such as slowing down the vehicle or applying brakes.

What are types of sight distance?

Overtaking Sight Distance

Overtaking sight distance means a totally safe distance that a driver needs to pass a slow-moving vehicle by using road space of the opposite direction of travel.

Intermediate Sight Distance

The intermediate distance (ISD) is well-defined as double the SSD figure. It is normally required during night driving conditions.

Head Light Sight Distance

Headlight sight distance is defined as the distance, which is measured in the direction of the vehicle, between the driver's position and the farsighted object without disturbing the line of sight.

Sight distance at intersections

The sight distance at the junction is measured at 2.5m back from the Stop Line. From this position, the driver must be able to see all other arms up to point of Stop Lines of each arm. If the junction is a signal-controlled junction, then all the primary and secondary signals against that arm must be visible to the driver.

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What are the factors affecting sight distance?

The most important factor for highway designers is to provide a desirable minimum stopping sight distance along the highway at the design stage. The factors such as driver response time, vehicle response time, operating speed, and type of obstruction also play a very important role.

Driver response time

Driver response time is the time taken by the driver from the moment he sees the object to the point where the brakes are applied. The total response time can be divided into four parts according to PIEV theory. In practice, all of these times are often combined into a complete visual-response time that suits design purposes and simple scale.

Most studies show that drivers need about 1.5 to 2 seconds under normal circumstances. However, in view of the variable site conditions and adding a factor of safety, a higher value is normally recommended which is 2.5 seconds.

The speed of the vehicle

The speed of the vehicle greatly affects the distance of sight. If the driver increases the speed of the vehicle, then the driver would need more time to react and act to stop or slow down the vehicle. Therefore, it is important to note that as the speed of the vehicle increases, the sight distance requirement also increases.

Effective brakes

The effectiveness of the brakes depends on the age of the vehicle, the features of the vehicle, its maintenance regime, etc. If the brake effectiveness is 100%, then the vehicle will stop as soon as the brakes are applied. But in reality, it is not always possible to attain 100% brake efficacy. For a safe geometric design, it is recommended to assume that the vehicle works with 50% brake efficiency.

Friction between the road and vehicle tire

The friction between the road and the vehicle tire plays very importantly to decelerate the vehicle. High Friction Surface of the road or high PSV (Polished Stone Value) road surface layer assists vehicles to slow down with put skidding. Please refer to our blog post on High Friction Surface for details.

Road gradient.

The steep road gradient amplifies the vehicle speed; therefore, it is very important to consider Road gradients as an important factor for sight distance. For example, if the road is passing a sharp curve and the gradient is also steep, it would be important to provide very clear visibility along the bend, otherwise, it would create a blind spot for the accident.

How to calculate Sight Distance?

There is a word known as safe stopping distance and it is one of the most important steps of highway design. Drivers must have adequate time to respond to an emergency. Therefore, in the construction of a highway, a viewing distance at least equal to the safe stopping distance should be provided. The stopping sight distance is the number of remaining distances and the flight distance.

The lag range is the distance transmitted by the vehicle at the time of t response and is given by vt, when v is accelerated to m-sec2. Brake distance is the distance travelled by the vehicle while the brakes were being applied. With a limited road, this is achieved by measuring the work done to halt the vehicle and the electric power of the vehicle.

If F is the maximum emerging force and the flight distance is l, then the work performed against the collision in the vehicle is Fl = fWl, where W is the weight of the vehicle. UK DMRB CD109 provides Stopping Sight Distance figures for various design speed situations.

Please see the picture below for details.

stopping sight distance

I hope the blog provides you with adequate information on Stopping Sight Distance. Please feel free to like, share and comment.

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What is Stopping Sight Distance? 6 Important Points